Further measurements will be made of the activities of the glutathione peroxidase system in various tissues of rats and chicks in response to dietary selenium. Studies will be continued on purification of glutathione peroxidase from rat lung and liver, and on properties of the enzyme. In vitro studies with a reconstituted glutathione peroxidase system in the presence of oxidizing emulsified arachidonate will test the pathway of reactions in which this protective enzyme is involved. The effects of dietary selenium and linoleate peroxide on the intestinal glutathione peroxidase system will be evaluated.